Engines may be configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. This operation may be referred to as VDE (variable displacement engine) operation. In some examples, a portion of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A control system may disable selected cylinders through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves. By reducing displacement under low torque request situations, the engine is operated at a higher manifold pressure, reducing engine friction due to pumping, and resulting in reduced fuel consumption.
However, a potential issue with variable displacement engines may occur when transitioning between the various displacement modes, for example, when transitioning from a non-VDE (or full-cylinder) mode to a VDE (or reduced cylinder) mode, and vice-versa. Specifically, the transitions can significantly affect the manifold pressure, engine airflow, engine power, and engine torque output. In one example, these transitions may produce disturbances in engine torque output and may increase noise, vibration, and harshness (NVH) of the engine.
The inventors herein have recognized the above issues and identified an approach to at least partially address these issues. In one example approach a method for an engine is provided comprising, during a first variable displacement engine mode in an engine having four cylinders, deactivating a first cylinder of the four cylinders and firing a second, third, and fourth cylinder of the four cylinders, each firing event separated by 240 degrees of crank angle (CA), and during a non-VDE mode, activating the first of the four cylinders and firing the first cylinder between the third and fourth cylinders. In this way, a four-cylinder engine may be operated in a full-cylinder or a three-cylinder VDE mode with even firing in the VDE mode.
In one example, a four-cylinder engine may include a crankshaft configured to fire three of the four cylinders at 240 crank angle degree intervals and fire the remaining cylinder of the four cylinders midway between two of the three cylinders being fired 240 crank angle degrees apart. An example firing sequence may include firing a first cylinder, firing a third cylinder at about 120 crank angle degrees after firing the first cylinder, firing a second cylinder at about 240 crank angle degrees after firing the third cylinder, and firing a fourth cylinder at about 240 crank angle degrees after firing the second cylinder, and firing the first cylinder at about 120 crank angle degrees after firing the fourth cylinder. Thus, the first cylinder may be fired at about 120 crank angle degrees between the fourth cylinder and the third cylinder and the second cylinder may be fired at 240 crank angle degrees after the third cylinder is fired and 240 crank angle degrees before the fourth cylinder is fired. Herein, firing events may occur at uneven intervals. The engine may also be operated in a three-cylinder mode wherein the first cylinder is deactivated, and the second, third and fourth cylinders are fired at about 240 crank angle degree intervals from each other. Additionally, the engine may be operated in a two-cylinder mode by deactivating two cylinders and firing the remaining two cylinders 360 crank angle degrees apart from each other.
In this way, an engine may be operated in one of two available reduced cylinder modes, each mode with even firing intervals between the firing cylinders. By operating the engine in the two-cylinder or three-cylinder VDE modes, a reduction in fuel consumption may be achieved. Further, since firing events in the two VDE modes occur at even intervals, NVH issues may also be diminished. The engine may be operated in a full-cylinder mode during high engine load conditions which may occur infrequently. Accordingly, the engine may largely operate in one of the two VDE modes resulting in an improvement in fuel economy as well as improved drivability. Overall, engine operational costs may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.